1. Field of the Invention
The present invention relates to a golf ball having a core, an inner cover formed around the core, and an outer cover formed around the inner cover, and more particularly to a golf ball capable of providing a high trajectory while maintaining high resilience and capable of providing a good hit feel.
2. Description of the Related Art
In general, the launching angle of a golf ball increases with the degree of softness of the golf ball. Therefore, in order to fabricate golf balls which provide a high launching angle and a high trajectory, there has been employed a method in which at least one of the layers of a golf ball is formed of a soft material; e.g., the core is formed of a relatively soft rubber, or one of the inner and outer covers (which may be referred as an intermediate layer and a cover) is formed of a relatively soft resin.
However, in the method in which the core is formed of a relatively soft rubber, when the core is rendered excessively soft, the golf ball becomes so soft that the resilience of the golf ball decreases, resulting in decreased travel distance, although the golf ball can provide a high trajectory. Further, when a player hits such a golf ball with a full shot, the player is given a feel that the ball has no core. Moreover, when the golf ball is hit by use of a putter, the golf ball generates a low-frequency sound, which gives many players an unfavorable impression.
In the method in which either the inner cover or the outer cover is formed of a soft resin, when the resin used to form the soft resin layer is excessively soft or when the soft resin layer is excessively thick, the golf ball becomes so soft that the resilience of the golf ball decreases, resulting in decreased travel distance, although the golf ball can provide a high trajectory. Further, when a player hits such a golf ball with a full shot, the player is given a feel that the ball has no core.
In view of the foregoing, an object of the present invention is to provide a golf ball which provides a high launch angle while maintaining high resilience and which gives a player who hits the ball with a full shot a feel that the ball has a core.
In order to achieve the above object, the present invention provides a golf ball having a core, an inner cover formed around the core, and an outer cover formed around the inner cover, wherein a plurality of through-holes are formed in the inner cover; projections are formed on the inner surface of the outer cover at positions corresponding to the through-holes; and the inner cover and the outer cover are joined together in a state in which the projections are received in the through-holes. The ratio between an area a of the core-side end surface of each projection and an area A of the outer-cover-side end surface of the projection; i.e., the ratio a/A, falls within the range of 0.04 to 0.80, and the ratio between a sum s of the areas A of the outer-cover-side end surfaces of the projections and a surface area S of a virtual spherical surface defined by the outer surface of the inner cover; i.e., the ratio s/S, falls within the range of 0.05 to 0.75.
In the golf ball of the present invention, when the hardness of the outer cover is rendered greater than that of the inner cover, the projections formed on the harder outer cover enter the softer inner cover, resulting in establishment of a state in which hard portions (projections) are embedded in the softer inner cover (hereinafter, the layer including the inner cover and the projections embedded in the inner cover may be referred to as an xe2x80x9cintermediate layerxe2x80x9d). By virtue of the above-described structure, the intermediate layer can provide a function as a soft layer (hereinafter referred to as a xe2x80x9csoft layer functionxe2x80x9d) and a function as a hard layer (hereinafter referred to as a xe2x80x9chard layer functionxe2x80x9d).
When a player hits the golf ball of the present invention with a full shot by use of, for example, a driver or long iron, a considerably strong external force is applied to the golf ball. In this case, the soft layer function of the intermediate layer attains a high lunching angle and low degree of spin. In addition, the projections produce a reaction force against the external force applied to the golf ball, so that the intermediate layer provides a hard layer function. Therefore, sufficient degree of resilience is obtained, and the player can sense presence of the core.
When a player performs a control shot for the golf ball of the present invention by use of, for example, a middle iron, an external force weaker than that in the case of full shot is applied to the golf ball. In this case, the reaction force generated in the projections becomes weaker as compared with the case of a full shot, so that soft hit feel is obtained. However, since the intermediate layer still retains the hard layer function to a considerable degree, a sufficient degree of resilience is obtained.
When a player performs a control shot for the golf ball of the present invention by use of, for example, a short iron, an external force weaker than that in the case of the control shot by the middle iron is applied to the golf ball. In this case, the reaction force generated in the projections becomes further weaker, so the intermediate layer does not provide the hard layer function. Therefore, soft hit feel and high degree of spin are obtained.
In the golf ball of the present invention, each projection is tapered such that the area of the core-side end surface of each projection is render smaller than that of the area of the outer-cover-side end surface (cross sectional surface) of the projection. Therefore, external force applied to the golf ball is properly received by the projections, so that high degree of resilience can be obtained effectively. Since the tapered shape of the projections decreases the contact area between the core and the projections, and the degree of softness of the inner cover increases toward the center of the golf ball, so that deformation of the projections in the shearing direction (tangential direction) occurs easily. Therefore, when a player takes a shot, such as a shot for applying considerable backspin to the golf ball, in which an external force is applied to the golf ball along the tangential direction, the projections deform easily in the shearing direction, so that soft hit feel and a high degree of spin can be obtained effectively.
As described above, the golf balls of the present invention can provide a high launch angle while maintaining high resilience and can give a player who hits the ball with a full shot a sensation that the ball has a core.